Work Page 168. What is work? A little different from what we’re use to in our everyday lives When...
-
Upload
morgan-parker -
Category
Documents
-
view
213 -
download
0
Transcript of Work Page 168. What is work? A little different from what we’re use to in our everyday lives When...
![Page 1: Work Page 168. What is work? A little different from what we’re use to in our everyday lives When you hear the word work, what do you think of?](https://reader036.fdocuments.net/reader036/viewer/2022082820/56649e965503460f94b99dab/html5/thumbnails/1.jpg)
Work
Page 168
![Page 2: Work Page 168. What is work? A little different from what we’re use to in our everyday lives When you hear the word work, what do you think of?](https://reader036.fdocuments.net/reader036/viewer/2022082820/56649e965503460f94b99dab/html5/thumbnails/2.jpg)
What is work?
A little different from what we’re use to in our everyday lives
When you hear the word work, what do you think of?
![Page 3: Work Page 168. What is work? A little different from what we’re use to in our everyday lives When you hear the word work, what do you think of?](https://reader036.fdocuments.net/reader036/viewer/2022082820/56649e965503460f94b99dab/html5/thumbnails/3.jpg)
Scientific Definition of Work
A force that causes a displacement on an object does work on the objectRemember displacement … change in
position (∆X = xfinal – xinitial) Work = force x displacement
W = Fd Work is not done on an object unless
the object is moved because of the action of the force
![Page 4: Work Page 168. What is work? A little different from what we’re use to in our everyday lives When you hear the word work, what do you think of?](https://reader036.fdocuments.net/reader036/viewer/2022082820/56649e965503460f94b99dab/html5/thumbnails/4.jpg)
An example
Is work done in the following scenario?1. A student holds a heavy chair at
arm’s length for several minutes.• No … why?
![Page 5: Work Page 168. What is work? A little different from what we’re use to in our everyday lives When you hear the word work, what do you think of?](https://reader036.fdocuments.net/reader036/viewer/2022082820/56649e965503460f94b99dab/html5/thumbnails/5.jpg)
Parallel Components
Work is done only when components of a force are parallel to a displacement When the force on an object and the object’s
displacement are in different directions, only the component of the force that is in the direction of the object’s displacement does the work
Components of the force perpendicular to a displacement does not do work
![Page 6: Work Page 168. What is work? A little different from what we’re use to in our everyday lives When you hear the word work, what do you think of?](https://reader036.fdocuments.net/reader036/viewer/2022082820/56649e965503460f94b99dab/html5/thumbnails/6.jpg)
For example …
Imagine pushing a crate along the ground
If the force you exert is horizontal, all of your effort moves the crate
If your force is other than horizontal, only the horizontal component of your applied force causes a displacement and does work
See Figure 5.2 page 169
![Page 7: Work Page 168. What is work? A little different from what we’re use to in our everyday lives When you hear the word work, what do you think of?](https://reader036.fdocuments.net/reader036/viewer/2022082820/56649e965503460f94b99dab/html5/thumbnails/7.jpg)
Angles …
Look at Figure 5.2 page 169 W = Fd (cos Θ) If Θ = 0, the cos(0) = 1
Therefore, W = Fd If Θ = 90, cos (90) = 0, so work =
0Remember, work is only done if
force is parallel
![Page 8: Work Page 168. What is work? A little different from what we’re use to in our everyday lives When you hear the word work, what do you think of?](https://reader036.fdocuments.net/reader036/viewer/2022082820/56649e965503460f94b99dab/html5/thumbnails/8.jpg)
Another example
Is work done in the following scenario?
1. A student carries a bucket of water along a horizontal path while walking at constant velocity.
• No … why?
![Page 9: Work Page 168. What is work? A little different from what we’re use to in our everyday lives When you hear the word work, what do you think of?](https://reader036.fdocuments.net/reader036/viewer/2022082820/56649e965503460f94b99dab/html5/thumbnails/9.jpg)
What if many forces are acting on the object?
Then, Wnet = Fnetd (cos Θ)
Net work = net force x displacement x cosine of angle between them
![Page 10: Work Page 168. What is work? A little different from what we’re use to in our everyday lives When you hear the word work, what do you think of?](https://reader036.fdocuments.net/reader036/viewer/2022082820/56649e965503460f94b99dab/html5/thumbnails/10.jpg)
One last thing …
UNITS!! Work has dimensions of force x length In the SI system, newtons x meters = joules (J)
Unit for work is the joule
![Page 11: Work Page 168. What is work? A little different from what we’re use to in our everyday lives When you hear the word work, what do you think of?](https://reader036.fdocuments.net/reader036/viewer/2022082820/56649e965503460f94b99dab/html5/thumbnails/11.jpg)
Let’s check your assignment from yesterday (#1-4 page 171)
1. For each of the following statements, identify whether the everyday or the scientific meaning of work is intended. A. Jack had to work against time as the
deadline neared. B. Jill had to work on her homework before
she went to bed. C. Jack did work carrying the pail of water up
the hill.
![Page 12: Work Page 168. What is work? A little different from what we’re use to in our everyday lives When you hear the word work, what do you think of?](https://reader036.fdocuments.net/reader036/viewer/2022082820/56649e965503460f94b99dab/html5/thumbnails/12.jpg)
Answers
1. A. Everyday senseB. Everyday senseC. Scientific sense
![Page 13: Work Page 168. What is work? A little different from what we’re use to in our everyday lives When you hear the word work, what do you think of?](https://reader036.fdocuments.net/reader036/viewer/2022082820/56649e965503460f94b99dab/html5/thumbnails/13.jpg)
#2
If a neighbor pushes a lawnmower four times as far as you do but exerts only half the force, which one of you does more work and by how much?
![Page 14: Work Page 168. What is work? A little different from what we’re use to in our everyday lives When you hear the word work, what do you think of?](https://reader036.fdocuments.net/reader036/viewer/2022082820/56649e965503460f94b99dab/html5/thumbnails/14.jpg)
Answer
2. The neighbor; twice as much
![Page 15: Work Page 168. What is work? A little different from what we’re use to in our everyday lives When you hear the word work, what do you think of?](https://reader036.fdocuments.net/reader036/viewer/2022082820/56649e965503460f94b99dab/html5/thumbnails/15.jpg)
#3
For each of the following cases, indicate whether the work done on the second object in each example will have a positive or negative value. A. the road exerts a friction force on a
speeding car skidding to a stop. B. A rope exerts a force on a bucket as the
bucket is raised up a well. C. Air exerts a force on a parachute as the
parachutist slowly falls to Earth.
![Page 16: Work Page 168. What is work? A little different from what we’re use to in our everyday lives When you hear the word work, what do you think of?](https://reader036.fdocuments.net/reader036/viewer/2022082820/56649e965503460f94b99dab/html5/thumbnails/16.jpg)
Answer
3. A. NegativeB. PositiveC. Negative
![Page 17: Work Page 168. What is work? A little different from what we’re use to in our everyday lives When you hear the word work, what do you think of?](https://reader036.fdocuments.net/reader036/viewer/2022082820/56649e965503460f94b99dab/html5/thumbnails/17.jpg)
#4
Determine whether work is being done in each of the following examples: A. a train engine pulling a loaded
boxcar initially at rest.B. a tug of war that is evenly matched.C. a crane lifting a car
![Page 18: Work Page 168. What is work? A little different from what we’re use to in our everyday lives When you hear the word work, what do you think of?](https://reader036.fdocuments.net/reader036/viewer/2022082820/56649e965503460f94b99dab/html5/thumbnails/18.jpg)
Answer
4. A. YesB. NoC. Yes